Heat-sensitive copying paper



Dec. 22, 1953 c. s. MILLER ETAL 2,663,657l

HEAT-SENSITIVE COPYING PAPER Filed May 15, 1952 2 Sheets-Sheet 1 /5765//77//2 60,0% f4 [Maerz/507:5

Z5 j; 07h/ Q Dec. 22, 1953 Filed May l5, 1952 c. s. MILLER ET AL HEAT-SENSITIVE COPYING PAPER 2 Sheets-Sheet 2 Patented Dec. 22, 1953 HEAT-SENSITIVE COPYING PAPER Carl S. Miller and Bryce L. Clark, St. Paul, Minn.,

assignors to Minnesota Mining & Manufacturing Company, St. Paul, Minn., a, corporation of Delaware Application May 15, 1952, Serial N0. 288,006

12 Claims.

This invention is concerned with duplicator sheet material or copying-paper useful in preparing copies of printed matter of the like, and with compositions and methods related thereto. The invention particularly contemplates heat-sensiftive coatings and coated sheet materials of a novel type, and methods of utilizing such materials in preparing duplicate copies of printed or other graphic subject-matter by simple exposure to high intensity irradiation and without subsequent developing of fixing operations.

This application is a continuation-in-part of our copending application Serial No. 747,339, iiled May 10, 1947.

Y Prior art copying-papers of which we are aware, which are subjected to exposure to visible light or similar forms of radiant energy in the preparation'of copies of graphicsubject-matter, must be protected from light before use, and require Various processing steps, in addition to the initial controlled exposure, for the production of visible light-stable copies.

Our new heat-sensitive copying-paper, on the contrary, is not itself affected by radiant energy such as visible or ultra-violet light, and may therefore be left exposed to such radiations both before and after printing. Nevertheless, radiant energy may be employed as the sole agency for the production of visible light-stable and permanent copies of graphic subject-matter on our novel copying-paper, or on other surfaces carrying our novel heat-sensitive compositions, by processes as hereinafter set forth.

The processes by which we are enabled to achieve this result, e. g. in the preparation of copies of the printed pages of books or the like, involve the irradiation of the printed surface, the resulant formation of an elevated-temperature pattern corresponding to the graphic subjectmatter of such printed surface, and the ultilization of this elevated-temperature pattern in the development of a facsimile copy of the graphic subject-matter on the heat-sensitive copyingpaper. i

Formulas may be'found in the prior art for various heat-sensitive paints, coated sheet materials, and the like. All such products of which we are aware exhibit one or more significant deiicencies when compared with our novel heatsensitive compositions and coated products. They f produce a visible change of suffi-cient intensity.

The change obtained at elevated temperatures may be reversible on cooling, i. e. the product ex.- hbits thermotropy. These and other deficiencies of prior art materials are avoided by the use of our novel compositions and products, particularly in connection with the copying of printed matter such as books, drawings or diagrams, pictures, etc. by means of our novel lcopying-paper herein described and illustrated.

The heat-sensitive compositions of our invention are comprised of solid ionizable reactants of which at least one is an electron donor component and another an electron acceptor component, which are potentially chemically capable of irreversibly and rapidly reacting at normal room temperature in a direct ion-exchange electron acceptor-donor chemical reaction to produce a visibly different reaction product, but which are normally maintained in physically distinct relationship and physically prevented from so reacting. The structure is so designed that an increase in temperature to a predetermined level allows the reaction to take place. The reaction is believed to be initiated by the melting, or soiening, or other physical change, of one or more of the reactive substances. Whatever the specific physical change employed, the application of heat results in an immediate reaction of the reactants and formation of a colored, opaque, or otherwise visibly different reaction product.

The rapid rate of reaction obtainable by this lmeans is particularly advantageous where the reactive material in sheet form is to be used as a heat-sensitive copying-paper. For the most effective reproduction of drawings or the like containing ne lines as well as massive dark areas, a high contrast number, as hereinafter dened, must be obtained when the sheet is heated through a narrow range of temperature not greater than about 207 C. and preferably about 5 C., within a period of time not greater than one second, and preferably within about one-tenth of a second, in order to obtain desirable contrast and detail.

Where these limitations are exceeded, the reproductions obtained may be blurred, fuzzy, or otherwise of poor quality. For example, a heatsensitive sheet material which darkens over a wide range of temperature, e. g. which provides a contrast number of 0.2 when heated to 70 C. but does not reach a contrast number of 0.8 until heated to 110 C., will ordinarily not produce a print having good contrast between light and dark areas.

A convenient method of determining the rate of reaction as well as the required temperature of activation for a particular copying-paper involves placing portions of the sheet against a series of metal surfaces each at a different but controlled temperature, and for various intervals of time. Our preferred copying-papers, when so tested, show a reaction from the original state to a color intensity or opacity equivalent to a contrast number of at least about 0.4, and preferably about 0.9, when heated at the required temperature level and within a temperature range of not more than about 20 C., and within a contact time of not more than about one-tenth second.

In determining the contrast number of a copying-paper, suitably exposed samples having printed (darkened) and imprinted (undarkened) areas are held against a flat magnesium carbonate block (a standard magnesium carbonate surface as defined in the paint industry). A beam of white light (north exposure daylight) is divolved, but are not to be construed as limitative.

EXAlVIPLE l Reactant A.To an aqueous solution of three mols of the sodium soap of commercial triplepressed stearic acid of melting-point approximately 53 C., and which supposedly contains a minor amount of other higher fatty acids, etc., add an aqueous solution of one mol of ferrie sulfate. Filter the precipitated ferrie stearato, wash separately with water and alcohol, and dry at room temperature. or melts within the range of 'Y0-95 C. Disperse 500 grams of the powder in two liters of denatured ethyl alcohol, e. g. Ponsolve, by grinding with flint pebbles in a ball mill.

'- Reactant B Dissolve 1500 grams of hexamethylenetetramine in 12 liters of denatured alcohol (Ponsolve) at 65 C.; with constant stirring, add this solution to a solution of 2000 grams of pyrogallic acid in 4 liters of the alcohol, also at 65 C. A crystalline precipitate forms. Cool to room temperture, with constant stirring to avoid lumping. Recover and dry the precipitate, which should be in the form of a fine powder. Disperse 560 grams of this powder in one and one-half liters of denatured alcohol by ball milling as for reactant A.

Reactcmt C.-Dissolve 10 grams of oxalic acid in 100 milliliters of ethyl alcohol.

Binder.-Dissolve 150 grams of polyvinyl butyral in one liter of ethyl alcohol.

Composition in liquid form The solid product softens In mixing and temporarily maintaining the `mixture of reactants A and B in a volatile vehicle such as denatured ethyl alcohol, a slight discoloration may sometimes be noted. This is presumably due to solution of traces of one or both of the substantially insoluble reactants in the liquid vehicle and the resultant reaction of the dissolved materials to produce a dark-colored reaction product.` The presence of a trace of oxalic acid, which forms a complex with iron and consequently may be considered to render inaccessible any dissolved or previously reacted iron, discharges and/cr prevents the formation of the slight discoloration thus otherwise produced. Citric acid, which likewise forms an iron complex, is also effective. In many cases the discoloration produced, even in the absence of these modifying reactants, is so slight as not to be troublesome, particularly where adequate precautions are taken in preparing and in applying the temperature-sensitive composition.

Coated transparent sheet material The above liquid composition is uniformly spread, as by means of a spreader bar or knife coater, over one surface of a thin transparent film of plasticized regenerated cellulose (cellophane) to a thickness of approximately 3 mils, and is allowed to dry at room temperature. The resulting visibly transparent coated sheet product rapidly darkens when heated to or above about 80 C., and is suitable for use as a temperaturesensitive copying-paper.

The darkening here obtained is thought to result from a preliminary melting or softening of the ferrie stearato, making possible the mixing or co-dissclving of the two reactants and their resulting rapid reaction to form a dark-colored and opaque reaction product. rihis may be inferred from the fact that the darkening of the sheet is obtained on heating the copying-paper to a temperature closely approximating the melting or softening point of the particular ferric soap used. A similar color change may be produced at room temperature by adding a suitable solvent, e. g. benzol, to the otherwise non-reacting mixture of the two powders.

This reaction and rapid color change does not take place on exposure of the mixture, or of the coated sheet material, to radiant energy such as visible or infra-red light in the absence of the required temperature increase. Some darkening of the coated transparent sheet material as above prepared may be observed on prolonged exposure to light, and particularly to high intensities of actinic radiation.; but this is believed to EXABMDLE 2 To obtain a composition having improved aging characteristics and which will react when heated to a somewhat lower temperature, similarly prepared ferric myristate, made with commercial myristic acid, is substituted for the ferrie stearate of Example l, and a reaction product of hexamethylenetetramine and gallic acid (a substituted trihydroxy benzene) is substituted for the pyrogallic acid product there described.

RMN?

'5 ingredient is prepared as iollows.: 280 grams of hexame'thylenetetramine 'are dm- -solved in fone 'liter of water, .188 grams of galllc acid are vdissolved 2in one liter of ethyl falvcohol. Gnevolume of the `first solution is mixed Awith Vtwo 'volumes of the second, and the 'result- Iing precipitate iis then recovered and dried. -It is ground into heptane, by means of a ball mill,

to orm a '25 dispersion.

Vli'erric lmyristate Vsoftens :and melts at somewhat lower temperatures than the stearate. Since the myristate is somewhat soluble in alcohol, heptane is used as the liquid vehicle, which .necessitates the substitution, for lthe alcoholsoluble but heptane-insoluble ,polyvinyl butyral, of a lheptane-soluble binder. In a .specific example, natural rubber was used as the binder. "Heptane is .substantially a non-solvent for both ferric `myristate and the reaction product .of hexamethylenetetramine and vgallic acid. There is ordinarilly no visible .evidence of reaction of '.the two materials in yfreshly prepared mixtures in'hcptane, although reaction takes place rapidly at temperatures below that required .for activation of the coated copying-paper, and usually at or even below normal room temperatures, as `in 1the case of Ythe reactants of Example 1, in the presence of a suitable solvent such as benzene. 'The final composition Aof the fluid mass prepared according tothe above description and ready Afor coating is approximately as follows:

Parts by weight This composition was coated on a thinpa'per, i. e. an unfilled flax paper of one Amil thickness, such as is used for cigarette-paper. 'The 'mixture was applied by means of a knife-coater set to give a coating orince of approximately l3 mils, and was dried at room temperature. The coated and at least partially impregnated ysheet product was faintly tan in color and appeared semi-transparent. On heating .to about 70 C. it rapidly darkened and became opaque. Exposure to light .alone had substantially no eifect on the appearance of the unheated material.

mine-gallic acid reaction product in conjunction with ferrie stearate prepared from highly puried stearic acid.

EXAMPLE 3 Ferric vstearate in -5 times its weight of stearic acid was emulsiiied in a hot concentrated solution of glue in water. To the cooled solution was added a water solution of tannic acid. The resulting well-mixed composition was coated onpaper and dried at room temperature. The sheet was red- -Scribing or otherwise mechanically compacting or disrupt- 6 `but 'turned black Vwhen heated 'as in the preparation or copies of graphic subiectlmatter Vlay-our lnovel method.

EXANIPLE '4 Ferric stearate was dispersed in vacetone by ball milling as in the `previous examples, and apre- .formed Vsolution of ethyl cellulose vwas added, to a "nal formula of approximately 112% *ferrie stearate, 6% ethyl cellulose, and 82% acetone. The mixture was coated in a `thin uniform layer on one mil condenser tissue, a thin| tough paper, and dried at room temperature. The weight of the dry coating of ferrie s'tearate particles uniiormly dispersed in ethyl cellulose was about '0.'68 gram per square foot of surface.

Gallic acid, which is a solid at normal room temperature, was dissolved in an acetone solution of ethyl cellulose, to a nal formula of approximately 5% gallic acid, 9.5% ethyl cellulose, and 85.5% acetone. This solution was coated in a thin layer over the `dry ferr-ic stearato-ethyl cellulose layer, and the sheet 'again dried at room temperature. The dry Weight of the second coating was about 0.05 gram per square foot.

The completed sheet `material was useful as a copying-paper, showing high contrast and good detail in the copy. The ferrie stearato component softens or melts at 'Z0-95 C., and at vleast the major portion of the color change in the copyingsheet occurs over substantially the same temperature range.

EXAMPLE 5 Fifty parts by weight of .ferric 'stearate were dispersed in 1500 parts of acetone by ball milling. Separately, 100 parts of a phenolic resin were dispersed in 400 parts of acetone by similar procedure. The phenolic resin was a solid heatadvancing partial reaction product of par-atertiary-butylphenol and formaldehyde, melting at 144-158" F. (S2-72 C.). Such a resin is soluble in drying oils and aromatic hydrocarbons but insoluble in alcohols and only slightly soluble in acetone.

Two parts of the ferrie stearato dispersion were mixed with one part of .the resin dispersion, and the mixture was coated on thin paper and dried at room temperature. The resulting sheet material was useful as a heat-sensitive copyingpaper, but the heat-'sensitive coating had a tendency to stick to .contacting surfaces, particularly during the copying process. Substitution of a higher-melting heat-advancing phenolic resin for the resin hereinbefore described eliminates this tendency, resins melting above 120 C. providing fully non-sticking lsurfaces with, however, some reduction in contrast in the copy obtained. Sticking is also readily avoided, and with no significant reduction in contrast, by incorporating small amounts of a nlm-forming binder, such as ethyl cellulose, inthe mixture prior to coating.

Other combinations of solid visibly inter-reactive materials which have provided electlve heat-sensitive copying-papers when coated on various paper or film back'ings in the form of dispersions in solutions of film-forming binders include ferrie stearate-triethyl sulfonium tan nate; ferric stearato-cadmium tannate; Vand ferric stearate-ammonium salicylate. Ferric myristate, ferric palmitate, and Asimilar salts of other organic acids softening or melting at temperatures within the range of about 60-l20 may be substituted for the ferrie stearate in these 'com'- positions.

In these examples, the visible change obtained on activation of the coated materialjs the result of a combination between the iron of the ferric stearate or equivalent and the phenolic portion of pyrogallic or gallic acid, tannates, salicylates, or the like. A large number of other phenols which similarly provide colored reaction products with iron salt are listed by Wess and Brode in the Journal of the American Chemical Society, volume 56, at pages 1039 and 1040.

EXAMPLE One pound of nickel acetate (tetrahydrate) was dispersed in one liter of anhydrous butyl acetate by grinding in a ball mill. Similarly, one pound of calcium sulfide was ground into one literof anhydrous butyl acetate. A 30% solution of V2 second nitrocellulose in butyl acetate and benzol was used as the binder. The three components were proportioned as follows:

v Volumes Nickel acetate dispersion 20 Calcium sulfide dispersion .5 Binder 30 This mixture was applied in a thin layer to various supporting surfaces, e. g. cellophane or cellulose acetate film, paper, wood, leather, etc., and dried at room temperature. The dried film was faintly greenish-white in appearance. When heated, the lm rapidly became intensely Ablack and opaque.

The reactive ingredients of Example 6 are found to be inter-reactive'under proper conditions at normal room temperature, with the production of a dark-colored and opaque reaction product. For example, the addition of a small amount of water to a mixture of nickel acetate and` calcium sulde in powder form initiates the reaction between the two compounds.V It is believed that the reaction obtained in the coated sheet material is due to the increased vapor pressure, at an elevated temperature, of the water 'present in the hydrated salt, and/or the melting of the hydrous salt to a fluid state, resulting in a reactive mingling of the nickel compound and the sulde.

' EXAMPLE`7 strontium sulfide and nickel acetate tetrahydrate were ground up in a 21% solution of 5-second nitrocellulose in butyl acetate, inthe ratio of one gram each of insoluble strontium sulfide and nickel acetate tetrahydrate particles to three ymilliliters or" nitrocellulose solution. The mixture was coated on cellophane and dried at room temperature. The sheet was light gray in color, :changing to an intense black when briefly heated .to 115 C. It could be used as a heat-sensitive copying-sheet by methods described herein, producingcopies of good contrast and detail. Bariumsulde and nickel acetate tetrahydrate produce similar results. In a specific formulation, 20 ml. of a dispersion of one pound of the nickel acetate in 1000 ml. of vbutyl acetate, 50 ml. of a, 28% nitrocellulose solution in butyl acetate, and 10 ml. of a dispersion of one pound of barium sulfide in 1000 ml. of butyl acetate were mixed and coated on paper, and dried at room temperature. A-protective surface sizing of a solution of nitrocellulose in butyl acetate was applied over the heat-sensitive coating and dried. The finished copying-sheet was light yellowish green in color, and produced dark-colored brown-black copies when used as a heat-sensitive copyingpaper by methods above indicated. Tested on a `nitrocellulose binder.

EXAMPLE 8 Another combination of heavy metal salt and alkaline earth metal sulfide which has produced useful copies when employed in structures such as that of Examples 6 and 7 contains solid particles of nickel stearate and barium sulde in a The nickel stearate, prepared by precipitation from a mixture of aqueous solutions of nickel acetate and the sodium salt of commercial stearic acid, is a waxy `solid at temperatures below 60 C., and liquid at 1.20o C. The coated and dried sheet was dipped into a 4: 1 blend of naphtha and mineral oil and again dried; the residual mineral oil improved the heatconductivity properties of the sheet. On a metal test block, the sheet changed rapidly from iight lyellow to dark brown at about 11%D C.; it could be used in making effective facsimile copies of vtypewritten letters and the like by the methods herein described.

EXAMPLE 9 Vand dried at room temperatures, were each eil'ective as heat-sensitive copying-papers and produced the indicated color change at the temperature specied.

1. Nickel palmitate barium sulde-stearic acid; changed from light green to intense black at C. Calcium sulfide produceda light gray sheet with otherwise identical properties.

2. Nickel palmitate-bariurn suliide-palmitic acid; changed from light gray to intense black at 70 C.

3. Nickel caprylate calcium suliide-stearic acid; changed from gray to intense black at 60 C. Crotonic acid in place of 4stearic acid raised the conversion temperature to 72 C.

VMany other compounds in addition' to the several materials previously enumerated have additionally been found to prcmlucey adequate color diierences when combined asclosely spaced particles in a film-forming binder'asvdescribed in the above Examples `6 9. Cobalt andlead salts are, for example, equally as eiiectivek as the salts of nickel.- Salts of silver, mercury, copper and iron are useful but less desirable, silverv and mercury salts showing a tendency to darken on exposure of the paper to sunlightv and copper and iron salts being initially undesirablydark in color and hence providing less contrast in the resulting copy. Colorless or lightly colored suldes suchas zinc suliide, cadmium sulfide, and magnesium sulfide are effective' in formulations such as those of Example 9 and for such'purposes may be considered fully equivalent to the alkaline earth metal sulfdes specified in thatv example.

eveeeiee? Commercial octadecyl amine was dissolved as the soluble hydrochloride by heating in water and adding hydrochloric acid until the oily Inaterial was dissolved. To this was added a saturated aqueous solution of potassium ferrocyanide. The yellowish precipitate of octadecyl ferrocyanide formed by this procedure was recovered, washed, dried, and dispersed in ay nitrocellulose lacquer in a'mixture of butyl acetate andV toluene. rlvvice this amount of ferrie steaf rate, prepared as described in connection with Example `l, was separately dispersed in a porgtion of the lacquer. TheY nitrocellulose amounted to about one-fourth the total Weight of the ferrie stearato and the octadeyl ierrocyanide when the'two dispersione were mixed.

The'mixture was coated on thin paper and dried at room temperature. The @Dated sheet was light gray, becoming dark blue when heated to about 80.? C., and producing clear and distinct thermo'copies of typewritten letters and the like.

A somewhat similar product was prepared with dispersione, in an alcohol solution containing polyvinyl butyral binder and a small amount of benzoic acid, of ferrie stearato and of the alcohol-insoluble reaction product of octyl amine and potassium ferricyanide. The dried coated sheet Was light yellowish green in color, changf ing to an intense blue when heated to about 9026-.

EXAMPLE 1l Lead myr-istate was prepared by adding an aqueous Solution ef leedeeeltele le en aqueous V`useful as a heat-sensitive copyi'rigpaper.

EXAMPLE i2 A dispersion was prepared, b y ball milling, rof .190 referer Weiehelef Cadmium steal-ate .in ,5.90 Yleegte ,Off heptane! The eedmum'eteeriate prepared by precipitation freni@ solution Of .sodium seep elf triple-Dressed .e'eere'laed- Seelerately, .a dispersion 1of`y2j0 parte 'Qfiphmu voarbazongin 20'()v parts Vof heptane, and a 'on'. tionl of parts of crepe rubberv in l90 par heptane, were provided. The tu@ 'die Siehe and the solution weremixed vin the vWeight ratio .of 17:2:3 to provide a liquid mixture VA'Wliieh `was coated 'onto .Hex tssuejend dried at room tem- .iierature The .eheetwas yellowish een', rapidly asins to red ,at approximately 1.00 5C# ed ,edQs useful heet-Sensitive Copying-palier.

EXAMPLE-13 Mercurio stearate kvvas submitted for the cadmium stearate 'and S-diphenyl carbazine for ythe diphenyl carbazone of Example 12,'the remaining ingredients and the relative proportions emaining the same. The coated .sheet Was light gray, changing Arapidly to purple when heated Lto approximately 90" C., and could be used to s'sue and f provide copies of printed mattei' by thermoeeprieel mehodsherem described l EXeNgLE 14 Cupric stearate replaced the cadmium stearato of-`Example"12 'in this example. "Additionally, there 'was addedto the mixture a small amount of sodium iodide, -to prevent prematurereaction and reductionoi contrast'. The 'sheet was" light tanv i-'n color; changing rapidly to purple when ieate'd to" 7'5-85 'C.`,"ad produced thermocopie's having adequate contrast and detail.

EXAMPLE 15 Thirty parts by weight of a dispersion ofdi phenyl carbazone in ten times its Weight of heptane were mixed With 5 parts of a 5% solution oi' 'rubber in heptane and coated in a thin layer on ilax4 tissue. Over the dried coating there was then applied a further coating of a mixture of 3'0 parts of a dispersion of zinc stearato in 5 times its weight of heptane, and 5 parts of the 5% rubber solution. The coated and dried sheet appeared snow white; it printed to a scarlet image at approximately C.

The reactions employed in Examples 11-15 will be recognized as typical color reactions for the detection of the particular polyvalent metal ion employed. Many other reactions having similar applications are well known, having been published, for example,"by Dr. F. Feigl in his' book, Qualitative Analysis Vby Spot Tests. The reactants selected must be solid at room temperatures, must be visibly inter-reactive at roomtemperatures when properly combined as by means of a mutual solvent, and at least one of the reactants must be fusible at a temperature within the range of about 60-120o C. The polyvalent metal ion is preferably introduced as the fusible salt of an ,I organic acid, as shown in Examples 11-15; but

other metallic compounds may be substituted where a fusible organic spot-test reagent, such as 2-ntroresorcinol `(M'. 83-85 C.) or 8hydroxy quinoline (M 7S-76 Q), is to be used. Strongly colored, odorous, volatile, or light-sensitive reagents are generally undesirable but may be found useful for special purposes, particularly when introduced in the inner layer of structures as describedExample 15. Methyl cellulose and polyvinyl butyral are examples of additional suitable binders.

The reactive solid components are conveniently applied to paper or other supporting structure as a. dispersion in a solution of a bonding agent in a suitable volatile vehicle, as disclosed in the above examples. The bonding agent assists in retaining the reactants on the surface of lthe support. However, 'other methods of applying the -reactants to the supporting surface and of maintaining them in proper relationship thereon may alternatively be employed. For example, a'polymerizable monomer may be substituted for the solution of bonding agent; after application, the monomer may vbepolyrnerized in situ to' form a.

binder nlm. The powdered reactants may be dispersed within, or on they surface of, a fibrous web or other ,supporting structure in the substantial absence ,of any added bonding agent. Additionally, the useof a nlm-forming bonding agent, such for example vas Athe.nitrocellulose of YExample 10 or other equivalent transparent nlm-.forming binder, `as a self-supporting lm as well as a lbinder and carrier for the reactive ingredients is also contemplated, Inthis type rof product, the film-forming composition containing the colorbination of bonding agents in significant proportions. The degree of contrast obtainable with j our copying-paper is readily controlled, for example, by suitably proportioning the relative `amounts of binder and of reactants. Thus, in-

creased contrast has been obtained by increasing the percentage of binder, and thereby presumably increasing the distance and the amount of nonreactive material between or around the individual particles of reactants. Conversely, a reduction in the percentage of bonding agent reduces the contrast and increases the detail obtainable in resultant copy.

Changes in the particle size and shape of any one or all of the reactant materials, and in the I'relative amounts of the individual reactants, will also have some eifect on the results obtained. li-"Jtoichiometric relationships between reactants may in any cases be determined, but are not necessarily most advantageous. The proportions `shown in the specic examples of the particular reactants therein disclosed have been found to produce good results, and may be used as a guide for the proper proportioning of othe reactants; but many variations will be found to produce acceptable products and are here contemplated. Where desired, various inert materials, such for example as pigments or the like, may be added to the sensitizing compositions of our invention. Additional surface coatings, e. g. of film-forming materials, may be applied as protective layers, or to impart desirable color, or for other purposes. Other modifications will be apparent.

In order to assure a clear understanding of the .structure of our novel product and the mannery in which it may be. utilized, e. g. in the copying of printed matter, reference is made togth'e accompanying drawing, in which:

Figure l is a conventionalized Aand, enlarged cross-section of one example of a suitable heatsensitive coating on a supporting member;

Figures 2, 3, and i are perspective views (partially out away) of three different arrangements lof a printed surface in relation to the coated surface of the copying-paper in obtaining facsimile reproductions of printed or other graphic matter `on the heat-sensitive coated copying-papers of lthis invention; and

. Figure is a diagrammatic, sectional view of one exampleA of structure, including a suitable source of radiantenergy, that may be employed vfor copying printed sheet material.

In Figure l, a heat-sensitive layer Iii, consisting of twol nely-divided reactantv materials II and I2, the individual solid particles of which lare containedv within a binder material I3, is sup'- ,ported on a supporting member I4.

In Figure 2, a transparent heat-sensitive copy'- -ing-paper I5, consisting of a heat-sensitive layer IB on a thin transparent backing I6(both shown Lft in cut-away section) is placed against-a-printed page Il', with the uncoated surface of the transaccess?? i'; parent backing I5 in contact with the printed characters I8 of which a facsimile copy is desired, The transparent backing I@ of Figure 2 is a specific member of the class more generically represented in Figure l by supporting member It. On exposure of the composite to intense illumination, as herein elsewhere explained, and in this case with the transparent heat-sensitive sheet I5 being between the source of light and the printed surface, a differential darkening, or other visible change, indicated by darkened area I9, is rapidly obtained in the heat-sensitive layer iii, corresponding to the printed character I8 therebelow.

The finished print obtained by the procedure outlined above in connection with Figure 2 is directly readable from the surface carrying the heat-sensitive layer. Transparency of the sheet material I5 to visible light is therefore not essential for subsequent reading of the copy obtained. Consequently the requirement of transparency here refers to the ability of the sheet to pass radiant energy of the wave-length employed in the copying process. The use of visible light thus imposes the requirement of visible transparency, as manifested, for example, in the construction embodying cellophane and described under Example l. With other wave-lengths, e. g. in the high-energy infra-red region, sheet material which is transparent to the infra-red but which-may appear translucent or even opaque to the eye may be used and is here contemplated.

In Figure 3, the heat-sensitive layer IIB of the (visibly) transparent heat-sensitive copyingpaper I5 is placed directly in contact with the printed Acharacters I8 of the printed page I'I. High intensity radiation directed against the transparent backing I6 results in a visible change, in the sensitive layer I0, corresponding to the printed characters I8, indicated by the darkened area I9 (partially concealed in the drawing). The resulting copy is a direct facsimile of the printed original when viewed through the visibly transparent backing IS.

In Figure Li, the heat-sensitive coating Ill is disposed, as shown in cut-away section, on a supporting member I4, which may be either transparent, translucent or opaque, but whichv should be at least reasonably non-conductive to heat. A thin opaque printed page 2li, which may or may not carry printed characters on the under side, but which has printed characters Iii, of which a copy is desired, on the outer side, is placed against the sensitive layer I0. Proper irradiation of the outer printed side of the printed page 2G results in the formation of a visible facsimile copy or darkened area I9, on the sensitive layer Ill, corresponding to the printed characters I8, With no undesirable blurring or ghost-image formation being obtained from any similar printed characters which may be present on the underside of the page 20. .y

In Figure 5, which illustrates in cross-section one means by which reproductions of printed matter may conveniently be made according yto the principles set forth in connection with Figure 2, an electrically-activated high-intensity source of useful radiation such as light rays 23,

comprising an incandescent line filament 2lv in transparent evacuated envelope 22, is shown supported within a movable trough-like reflector 2li of elliptical cross-section. Filament ZI is located at one focal line of the elliptical reflector; lconsequently light rays`23, emanating therefrom, are focused at the other focal line, which is caused to coincide with the printed surface of printed 13 page I1, carrying printed or other graphic' characters I8 of which a facsimile copy is desired.

Page I1 rests on the uncoated surface of a sheet of transparent heat-sensitive copying-paper I5, consisting of a transparent backing I6 and a heat-sensitive coating l0. The copying-paper rests on a transparent heat-insulating sheet material which may be an open-mesh screen such as silk bolting-cloth, and this screen in turn rests on a transparent support 26, which may be a glass plate, preferably of heat-resistant glass. (Where support 26 is inherently sufficiently heat-non-conductive or is suitably roughened or otherwise treated at the surface so as to present anon-conductive ysurface to the copyingpaper I5, the separate thickness of insulation 25 may be eliminated.) The copy-paper I5 and page I1 are vheld in close and heat-conductive contact by means of a vheat-insulating cover 21, which may bea wooden cover, as shown in Figure 5, or may be constructed of any other suitable material including a soft and flexible material such as a pneumatic or hydraulic pressure-bag. If desired, the cover 21, and the underlying printed page I1, may be maintained at any desired 'temperature below the printing temperature of the copying-paper I5, by suitable temperature control means. The elliptical reflector 24, with its enclosed light source, is movable across the surface of the thermographic assembly, as indicated by the double arrow 28, by suitable means (not shown), in a plane such that vthe light rays 23 are continuosly focused at the printed surface of the printed page I1. The rate of movement of the reflector assembly is adjustable, to compensate for differences in the thickness and heat conductivity of the page to be copied, here shown as printed page l1, and also to compensate for differences in the sensitivity of the temperature-sensitive layer I0, the intensity of light output, and various other factors.

For rapid copying of text-book pages, letters, or the like with the apparatus of Figure 5, it has been found convenient to use a 200G-watt coiled tungsten line-filament incandescent lamp, 10 inches long, with Pyrex glass envelope, mounted in an elliptical reflector 24. The resulting concentration of radiation is of such high intensity as to cause charring or burning if allowed to remain focused on the same section of paper for more than about one second. With this light source, and with temperature-sensitive copyingpaper such as that described under the various examples, clear unfogged facsimiles may be obtained. However, many other sources of radiation, including ordinary high-intensity incandescent lamps, photoash lamps, electric arc, infra-red lamps, concentrated sunlight, etc., have been found to be effective in various thermographic assemblies using our heat-sensitive copying-papers.

Many modifications, both in preparing the reactive compositions and the coated materials, and in utilizing the products in the direct reproduction of graphic subject-matter, have been described. While the importance of each of these and other modications in the commercial application of the principles involved will become obvious in view of the foregoing description, it will be even more apparent that the underlying principle, on which the novel phenomena herein presented are based, remains the same in all of the applications and modifications thereof. This principle involves the selection and blending of two or more solid reactants in such a of conditions, it has been shown to be particularly valuable in Ymaking available, as anew article of manufacture, a heat-sensitive copying paper suitable for the preparation of visible lightstable copies of graphic subject-matter by methods involving only the exposure of such matter to intense irradiation While in heat-conductive relationship with the heat-sensitive material.

Having now described our invention in terms of specific examples and embodiments, but without intent to be limited thereto, whatwe claim is as follows:

l. A heat-sensitive copying-sheet for making direct, high contrast, clear detail copies of graphic subject-painter as herein described, said copyingsheet being non-sticking at C., being stable at normal room and storage temperatures, and being rapidly and permanently visibly changed when heated from room temperature to 120 C.; said copying-sheet comprising a support having low thermal conductivity, and a Aheat-,sensitive iayer firmly bonded thereto and containing .a plurali ty of stable ionizable dissociable normally solid reactant materials including at least one 'electron donor component and at least one electron acceptor component disposed in contiguity with each other in said layer, at least one of said components being fusible at a temperature within the range of about 60-120 C. and being present in an amount such that on fusion and reaction with other components a visible change is produced in the heat-sensitive layer, said components being maintained in physically distinct and chemically inter-reactive relationship in said layer at normal room and storage temperatures, and said components being so selected as to be inter-reactive in a direct ion-exchange electron acceptordonor chemical reaction at room temperature in a mutual solvent capable of permitting ionization of said components and with the formation of a stable. visibly distinct, highly polarized compound which is less dissociable than either of said components.

2. A heat-sensitive copying-sheet for making direct, high contrast, clear detail copies of graphic subject-matter as herein described, said copying-sheet being non-sticking at 120 C., being stable at normal room and storage temperatures, and being rapidly and permanently visibly changed when heated from room temperature to 120 C.; said copying-sheet comprising a support having low thermal conductivity, and a heatsensitive layer containing, uniformly dispersed in a nlm-forming binder non-fusing at temperatures below about 120 C., a multitude of closely spaced solid particles of a plurality of stable ionizable dissociable normally solid reactant materials including at least one electron donor component and at least one electron acceptor component, at least one of said components being fusible at a temperature within the range of about 60-120 C. and being present in an amount such that on fusion and reaction with other components a Visible change is produced in the heat-sensitive layer, said components being maintained in physically distinct and chemically inter-reactive l5 relationship in said layer at normal room and storage temperatures, and said components being so selected as to be inter-reactive in a direct ionexchange electron acceptor-donor chemical reaction at room temperature in a mutual solvent capable of permitting ionization of said components and with the formation of a stable, visibly distinct, highly polarized compound Which is less. dissociable than either of said components.

3.The heat-sensitive copying-sheet of claim 1 wherein an electron acceptor component is an ionizable compound of an anion and a polyvalent metal cation, and an electron donor component is an ionizable organic reagent rapidly reactive with sai-d polyvalent metal cation to provide a visibly distinct reaction product.

4. The heat-sensitive copying-sheet of claim 2 wherein an electron acceptor component is an ionizable compound of an anion and a polyvalent metal cation, and an electron donor component is an ionizable organic reagent rapidly reactive with said polyvalent metal cation to provide a visibly distinct reaction product.

5. A copying-sheet according to claim l in which an electron acceptor component is an iron salt melting at a temperature Within the range of about 60-l20 C., and an electron donor component is a Water-insoluble organic compound having an iron cyanide complex ion.

6. A copying-sheet according to claim 5 in which the iron salt is a ferrie salt of a fatty acid.

7. A copying-sheet according to claim 6 in which the electron donor component is a waterinsoluble'organic 'reacton'product of an organic amine and ai Water-soluble compound having an iron cyanide complex ion.

'8. A copying-sheet according to claim 2 in which an electron acceptor component is an iron salt melting at a temperature Within the range of about 60120 C., and an electron donor component is a water-insoluble organic compound having an iron cyanide complex ion.

9. A copying-sheet according to claim 8 in which the iron salt is a ferrie salt of a fatty acid.

l0. A copying-sheet according to Aclaim 9 in which the' electron donor component is a Waterinsoluble organic reaction product of an organic i amine and a water-soluble compound having an iron'cyanide complex ion.

ll. AV copying-sheet according to claim 10 in Which the electron donor component is a Waterinsoluble organic reaction product of an alkyl amine and a water-soluble ferrocyanide.

12. A copying-sheet according to claim l0 in which the electron donor component is a waterinsoluble organic reaction product of an alkyl amine and a water-soluble ferricyanide.

CARL S. MILLER. BRYCE L. CLARK.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,844,199 Bicknell Feb. 9, 1932 2,378,585 Schroth June 19, 1945 

1. A HEAT-SENSITIVE COPYING-SHEET FOR MAKING DIRECT, HIGH CONTRACT, CLEAR DETAIL COPIES OF GRAPHIC SUBJECT-MATTER AS HEREIN DESCRIBED, SAID COPYINGSHEET BEING NON-STICKING AT 120* C. BEING STABLE AT NORMAL ROOM AND STORAGE TEMPERATURES, AND BEING RAPIDLY AND PERMANENTLY VISIBLY CHANGED WHEN HEATED FROM ROOM TEMPERATURES TO 120* C.; SAID COPYING-SHEET COMPRISING A SUPPORT HAVING LOW THERMAL CONDUCTIVITY, AND A HEAT-SENSITIVE LAYER FIRMLY BONDED THERETO AND CONTAINING A PLURALITY OF STABLE IONIZABLE DISSOCIABLE NORMALLY SOLID REACTANT MATERIALS INCLUDING AT LEAST ONE ELECTRON DONOR COMPONENT AND AT LEAST ONE ELECTRON ACCEPTOR COMPONENT DISPOSED IN CONTIGUITY WITH EACH OTHER IN SAID LAYER, AT LEAST ONE OF SAID COMPONENTS BEING FUSIBLE AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 60-120* C. AND BEING PRESENT IN AN AMOUNT SUCH THAT ON FUSION AND REACTION WITH OTHER COMPONENTS A VISIBLE CHANGE IS PRODUCED IN THE HEAT-SENSITIVE LAYER, SAID COMPONENTS BEING MAINTAINED IN PHYSICALLY DISTINCT AND CHEMICALLY INTER-REACTIVE REALATIONSHIP IN SAID LAYER AT NORMAL ROOM AND STORAGE TEMPERATURES, AND SAID COMPONENTS BEING SO SELECTED AS TO BE INTER-REACTIVE IN A DIRECT ION-EXCHANGE ELECTRON ACCEPTORDONOR CHEMICAL REACTION AT ROOM TEMPERATURE IN A MUTUAL SOLVENT CAPABLE OF PERMITTING IONIZATION OF SAID COMPONENTS AND WITH THE FORMATION OF A STABLE, VISIBLY DISTINCT, HIGHLY POLARIZED COMPOUND WHICH IS LESS DISSOCIABLE THAN EITHER OF SAID COMPONENTS. 